Protected homo-oligopeptide structure: Model for preferred conformation of a linear methionine heptapeptide in chloroform.

The (1)H nuclear magnetic resonance spectra for a protected linear heptapeptide of methionine, Boc-Met(7)-OMe, were measured in dimethyl-d(6) sulfoxide and C(2)HCl(3) solutions. In the former, the peptide is disordered. In C(2)HCl(3), preferred (short-range) interactions exist that dominate the overall secondary structure of the peptide. A model is proposed in which seven-membered ring structures are initiated at the NH(2)-terminus and propagate toward the carboxyl terminus. The first seven-membered ring contains a strong hydrogen bond between the amide NH of residue 2 and the carbonyl oxygen atom of the urethane blocking group. As one proceeds toward the carboxyl terminus the hydrogen bond strength of succeeding rings decreases, and at residue 5 the peptide chain assumes an extended structure that is stabilized by intermolecular hydrogen bonds. The model accounts for most of the results from (1)H NMR and infrared studies on the heptapeptide and is consistent with conformational energy calculations on homo-oligopeptides. It suggests that the COOH-terminal residues may serve as the nucleus for intermolecular peptide-peptide associations.